A position detecting sensor is attached to a fluid pressure cylinder to determine the operating position of a piston. The position detecting sensor is provided outside the cylinder to correspond to a magnet attached to the piston in the cylinder.
According to the forms of fluid pressure cylinders, conventional mechanisms for attaching a position detecting sensor include a rail-type mechanism in which a position detecting sensor is attached with a sensor holder to a rail disposed on the outer surface of a cylinder tube in the longitudinal direction, a tie-rod-type mechanism in which a sensor holder is attached using tie rods connecting the end covers of a cylinder tube, and a groove-type mechanism in which a position detecting sensor is attached directly into a groove provided on the outer surface of a cylinder tube.
Of these mechanisms, the use of the tie-rod-type mechanism is limited to fluid pressure cylinders having tie rods. For the groove-type mechanism, a plurality of position detecting sensors cannot be provided to a single groove such that they approach each other to a distance of not more than the length of the sensors. Accordingly, a plurality of grooves must be provided on the outer surface of a cylinder tube to dispose the sensors in close proximity. For a fluid pressure cylinder, generally, position sensing at the ends of a stroke is often demanded. If the cylinder has a long stroke compared to the length of position detecting sensors, the sensors may be easily attached. If, on the other hand, the cylinder has a short stroke, the position detecting sensors interfere physically with each other in the groove mechanism. For a compact cylinder, particularly, position detecting sensors cannot be attached because the placement of grooves is spatially limited.
For the rail-type mechanism, a position detecting sensor is attached outside the rail walls of a rail. This mechanism is therefore advantageous in that a plurality of position detecting sensors may be disposed on a single rail in close proximity using the outer surfaces of the pair of rail walls by modifying the structure of a sensor holder.
In the rail-type mechanism, the position detecting sensors may be attached directly to the rail. In this case, however, general-purpose position detecting sensors cannot be used, and position detecting sensors must be prepared which have a special attachment structure and strength against, for example, tension from a lead.
The above types of position detecting sensors for sensing the position of a piston are typified by reed switch sensors, which open and close a contact using a magnetic force, and electronic magnetic sensors, which include a magnetoresistive element and have no electrical contact.
In recent years, electronic position detecting sensors have often been used for their long life and environmental compatibility. The magnetoresistive element used detects and converts the amount of change in magnetic force into an electrical signal, and the operating position of a piston is determined according to the detection output. Since the magnetoresistive element itself has directivity in magnetic detection, it must be attached such that the central axis line of the directivity is directed toward the center of a cylinder.
For the above rail-type sensor attachment mechanism, however, a position detecting sensor is positioned outside the rail walls of a rail. If, therefore, the position detecting sensor is attached in a direction parallel to the line between the centers of the rail and a cylinder, the central axis line of directivity of the position detecting sensor is not directed to the center of the cylinder. In particular, if the cylinder tube has a small diameter, the central axis line of the directivity deviates significantly from the line between the centers of the rail and the cylinder.